DC/DC converters have been popular in industry for many years. Current mode DC/DC power converters include various designs which have been used. Such designs include fixed or constant frequency peak and valley current mode devices. These devices are turned on and off during each cycle at the fixed frequency of operation. There are devices that operate so that either the “on” time (of each cycle) remains constant (on time control), or the off time (of each cycle) remains constant (off time control), i.e., control the duty cycle by controlling the on time or off time of each cycle of control.
These conventional systems normally include an energy storage device, usually in the form of an inductor, so that energy can be stored during the on time of each cycle and used during the off time of each cycle. The converter system is regulated usually by sensing the current through the inductor, not the current through the load. If there is a transient in the load, i.e., it is drawing either more or less current than it was, the converter wants to keep the load voltage constant, and must respond to the transient as quickly as possible. Because the converter senses the current information, it can provide accurate protection against over-current conditions. Converters can be fixed frequency peak or valley current architectures. Further, current mode converters are easy to compensate to insure a stable output when load conditions are stable, and therefore they are easy to use. For example, constant frequency peak current mode controlled architecture provides a natural current limit and is stable over a wide range of input and output conditions. It is also ideally suited for multiphase applications where current sharing and transient response is of greater importance. Increasing current limit beyond maximum load current allows headroom for improved response during transients at heavy load. This headroom however increases the size and rating requirement of the converter power components and leads to a reduced signal to noise ratio under nominal conditions.
Since current mode controlled DC/DC converters limit the output current, they thus act as a current source. During an output load change, initially the current is provided by output capacitors because no converter is fast enough for a sudden and rapid load change. The output will rise/fall depending on the difference between load and inductor current. The controller will sense this change in voltage and turn on the appropriate switch. In the case of the constant frequency peak current mode controlled converter, the worse case happens when the output load increases right after the control switch has just turned off. The converter has to wait for the next clock pulse to turn on the control switch. For a sufficiently high bandwidth converter, this clock latency is the main reason for voltage droop during the transient response for this architecture. Similar latency issues exist in the other current mode architectures for different load transient conditions.